The gal operon of E. coli is a classic negative repression system that contains three genes which encode for enzymes required in galactose metabolism. GalR, the gal repressor, regulates transcription from this operon. Its function involves multiple linked equilibria including the basic repressor-inducer binding and repressor-operator interactions. A GalR monomer-dimer equilibrium and interactions with other regulatory proteins (Hu and cAMP-CRP) probably play important roles in in vivo function. The magnitude and linkage of these interactions can be probed in vitro by physical studies. Intrinsic tryptophan fluorescence was used to characterize GalR:D-galactose binding as a function of temperature. The binding appears cooperative, at least at low temperatures, and the estimated van't Hoff enthalpy ( HvH) is -2.5 kcal/mol. Direct calorimetric determination of the thermodynamic parameters for ligand binding to GalR will improve our understanding of the energetics and allosteric control in this system, and of transcriptional regulation in general.